The endogenous opioid system comprises three principal opioid receptor sites in the central nervous system and in the periphery designated as μ (Mu), κ (Kappa) and δ (Delta). The pharmacological response is elicited by binding of a multitude of endogenous opioid ligands to these receptors, the principal ones being the enkephalins, endorphins, and dynorphins. The exogenous opioids/opiates exert their activity by mimicking and/or antagonizing the activity of the endogenous opioid ligands at these receptors. Since the anatomical location, distribution and function of the opioid receptors is wide and varied (Neuropharmacology, 21, 487-497; Med. Res. Rev., 11, 357-374), the pharmacological effects elicited by their agonism and antagonism are diverse as well.
The μ receptors which bind morphine and its derivatives are responsible for analgesia, respiratory and gastrointestinal functions, sedation, neuroendocrine functions and mediate opiate dependence. The δ receptors are abundant in the CNS and mediate analgesia, feeding and various hormonal functions. The κ receptors are widely distributed in the CNS, peripheral tissues, nerve endings, immune cells, etc., and are responsible for functions including analgesia, gastrointestinal functions like food intake, gut motility, water balance, thermoregulation and various neuroendocrine functions. (J. Pharmacol. Exp. Ther. 234, 463-469; Peptides 4, 797-800; Goodman and Gilman's The Pharmacological Basis of Therapeutics (11th Edition) Chapter 21, Pp 547-590). It should further be noted that the κ receptors are also widely distributed in peripheral tissues, nerve endings and immune cells, etc.
Pharmacologic studies with receptor selective ligands have shown that analgesia can be produced by selective activation of each of the three types of opioid receptors. Most clinically used opioid analgesics, such as morphine and codeine act as μ receptor agonists. These opioids are known to feature undesirable and potentially dangerous dependence forming side effects. The κ opioid receptors, on the other hand have attracted special attention because their selective activation can produce analgesia without causing dependence and respiratory depression that is typically associated with μ receptor activation by morphine (Pharmaceutica Acta Helvetiae, 74, 2-3, Pp 337-344).
The opioid receptors are members of the superfamily of G-protein-coupled receptors (GPCRs). An agonist binding to the κ receptor activates the intracellularly associated G protein, which decreases Ca2+ channel conductance or inhibits adenylyl cyclase (AC). In addition to analgesia, potential applications of κ selective agonists include treating conditions, such as diuresis (Pharmacology Biochemistry and Behavior, 65, 1, Pp 53-59), eating disorders, motion sickness, and neuroprotection (Peptides 29, 12, Pp 2292-2375), among other disorders. Therefore, the κ receptors represent important therapeutic targets. Ligands selective for the κ receptors can serve as important pharmacologic tools. For example, such compounds can be used in competition assays to determine the relative specificity and selectivity of other compounds for the κ receptor, as well as for μ and δ receptors.
Various different classes of compounds featuring KOR agonist activity have been described in the art including the following:
U.S. Pat. No. 7,112,598 describes 2-phenylbenzothiazoline derivatives as KOR agonists.
U.S. Pat. No. 5,681,830 describes diarylmethyl piperazine compounds having utility as exogenous receptor combinant species for binding with opioid receptors such as kappa receptors.
European Patent No. 0 663 401 describes morphinan derivatives as selective KOR agonists and their application as an analgesic, diuretic, antitussive and brain cell protective agent.
The κ opioid receptor (also known as KOR) modulation has also been reported to be useful in the treatment of arthritis (Life Sciences, 57, 4, Pp 371-378), hypertension, pain, particularly pain which is inflammatory in origin and post-operative pain, (European Journal of Pharmacology, 429, 1-3, Pp 79-91) inflammation, migraine, inflammatory disorders of the gastrointestinal tract, psoriasis, and irritable bowel syndrome (IBS), Parkinsonism, (European Journal of Pharmacology, 396, 2-3, Pp 101-107, Molecular Brain Research, 44, 1, Pp 12-20) and stroke.